The above-referenced parent patent application discloses a digital video recorder which has intelligent video information management capabilities. A preferred application of the recorder of the parent application, as disclosed therein, is for selective recording of video signal streams generated by closed-circuit video security surveillance cameras. For this application, the recorder disclosed in the parent application is adapted to receive simultaneous input video signal streams from up to 16 cameras.
It is often desirable to record signal streams output from surveillance video cameras in order to preserve a record of unusual events captured by the video cameras. However, because of the enormous quantity of raw information present in a video signal stream, it is difficult to provide in an economical manner sufficient storage capacity for video signals corresponding to long periods of time. There have been a number of proposals intended to provide for efficient storage of surveillance video signals. For example, it has been proposed to record surveillance video signal streams only at very low frame rates (say, on the order of one field or frame per second). Low frame rate recording, however, carries the risk that crucial images will go unrecorded.
The difficulties in providing adequate storage capacity are increased when it is desired to have more than one video camera share a single recording device. One technique that has been utilized to improve the effectiveness of a shared recording device is to allocate larger portions of the recording bandwidth to video signal streams in which motion is detected. The premise is that signal streams representative of static images are unlikely to include information that is important. While this is a useful strategy, there remain significant risks that important information will be left out of the stream of images actually selected for recording.
The parent patent application discloses an additional strategy, referred to as pre-alarm buffering, which also promotes efficient use of limited signal storage capacity. The basic concept is that the respective video streams generated by some or all of the cameras connected to the recorder are not usually selected for permanent storage. Instead, a ring buffer is provided to record the signal streams at a given (perhaps shared) field rate for a limited period of time, say one minute. If a significant event is detected, such as tripping of an alarm sensor, actuation of an alarm condition by a human operator, or detection of a predetermined characteristic of an incoming image stream by machine analysis of the image stream, then the video signal stream in the ring buffer, which would normally have been overwritten, is preserved for permanent storage. Pre-alarm buffering represents a desirable compromise which avoids devoting permanent storage capacity to large quantities of uninteresting information, while increasing the likelihood that significant information will be permanently recorded. The present application is concerned with techniques for efficiently managing pre-alarm buffering operations.